The present invention relates in general to the field of household appliances, and more particularly to a heating circuit with monitoring arrangement for appliances like laundry washers, combined washers&dryers, dryers, dishwashers and the like, and in general for all those appliances wherein there is the necessity of heating a fluid (a washing liquid like in laundry washing machines or in dishwashers, or drying air like in laundry dryers).
Heating circuits for household appliances like those listed above generally comprise a fluid heating element, consisting of a heating resistor, and a switch element (e.g. a relay commanded by an appliance control unit, or a level switch which closes only when a sufficient amount of liquid is present in the washing tub to ensure that the heating resistor is fully immersed) for selectively energizing the heating resistor when required, for example in order to heat the washing liquid for washing laundry or dishes, or to heat the air flow used to dry the laundry.
The heating circuit is generally monitored for assessing the proper operation and detecting possible faults thereof. Faults may as a matter of fact occur in the heating resistor or in the switch element energizing it. The heating circuit should be monitored to identify whether the heating resistor is powered on or off, or if it is short-circuited to earth. Some of these faults may be extremely dangerous, for the appliance and even more for the user's health. For example, overheating of the heating resistor should be prevented, not to cause component parts to be damaged or destroyed, and fires to be produced; also, a heating resistor that happens to be short-circuited to earth is a source of danger, because dispersion currents may reach the appliance cabinet and cause electrical shocks to the user when touching it. In case a fault of this type is detected, a decision is to be taken to halt the appliance.
The Applicant has observed that known monitoring arrangements of the heating circuit are not capable of discriminating among all the possible different types of faults the heating circuit may suffer. The impossibility of discriminating the nature of the fault leads to classifying some faults as dangerous for the user's safety and thus lead to the appliance halt even if, actually, there would be no risk and the machine operation could be continued. This is undesirable, because the user has to wait for the intervention of the service personnel even if, in principle, the machine could continue to operate, although with lower performance.
The Applicant has also observed that some of the known solutions for monitoring the heating circuit cause power consumption even when the appliance is not operated (i.e., it is off). Also this is undesired, especially nowadays that the power consumption of household appliances is a major quality factor.